1. Sec. II – Chemical Reactions

2. In this section, we are going to be getting into chemical reactions.
Chemical reactions are all around us.
They can be external like the burning of fossil fuels (combustion reactions) or using epoxy cement to repair broken things.
Chemical reactions can also be found internally – digestion is considered a chemical reaction because chemical bonds are broken. Cellular respiration (the generation of energy) is also another important reaction.
The question is: How do we know a chemical reaction has occurred??
Through temperature changes
Most reactions can be grouped as those that give off heat (EXOTHERMIC) or those that take in heat from the environment (ENDOTHERMIC)
Through colour changes
Odours
Gas production
Precipitates formed
Irreversible changes

3. How do we write a reaction?
There are typically 2 parts to a chemical reaction:
REACTANTS  PRODUCTS
It’s important to note that reactants (what is combined together) are always written on the left side. The products (what is produced by combining the reactants) are always written on the right side.
Usually the 2 sides are joined by an arrow going from left to right (). This arrow shows the reaction only goes one way and can’t be reversed.
An example can either be written as a word equation.
Magnesium ribbon + Oxygen  Magnesium oxide
Or as a chemical equation using the proper symbols
Mg(s) + O(g)  MgO(s)

4. Symbols used in equations
Purpose +
Separates two or more reactants or products 
Separates reactants from products
 
Separates reactants from products and indicates a reversible reaction
(s)
Indicates a solid state
(l)
Indicates a liquid state
(g)
Indicates a gaseous state
(aq)
Indicates a water solution
It’s important to remember to include the states of the reactants and products when writing out a chemical equation.

5. Balancing chemical equations
When you first write out a chemical equation, it is considered to be a skeleton equation. The products and reactants are identified but not how much of the reactants and products take part.
This is where the idea of balancing a chemical equation comes into play. THE BALANCING OF CHEMICAL EQUATIONS IS A KEY CONCEPT IN CHEMISTRY AND HAS TO BE UNDERSTOOD REALLY REALLY REALLY WELL (get the idea it’s important?)

6. Frame + wheel + handlebar + pedal  bicycle reactants product
Suppose you were going to build bicycles. (we are going to simplify the build to just 4 components (reactants))
Frame + wheel + handlebar + pedal  bicycle
reactants product
Now that we have written out the skeleton word equation we need to determine how many of each reactant are we going to require to build our bicycle.
We signify the amount of each reactant by placing an appropriate coefficient in front of each reactant.
1 Frame + 2 wheels + 1 handlebar + 2 pedals  1 bicycle
To write this balanced word equation as a balanced chemical equation we would write the appropriate ‘chemical’ symbols for the parts.
F = frame W = wheel H = handlebar P = pedal.
1 F + 2 W + 1 H + 2 P = ?
Using the appropriate subscripts, the ’formula’ for the bicycle is
FW2HP2

7. Rules for writing and balancing equations
Determine the correct formulas for all the reactants and products.
Write the skeleton equation by placing the formulas for the reactants on the left and the formulas for the products on the right with a yields sign  in between. If two or more reactants or products are present, separate their formulas with plus signs.
Determine the number of atoms of each element in the reactants and products. Count a polyatomic ion as a single unit if it appears unchanged on both sides of the equation.
Balance the elements one at a time by using coefficients. When no coefficient is written, it assumed to be 1. Begin by balancing that appear only once on each side of the equation. Never balance an equation by changing the subscripts in a chemical formula!. Each substance has only one correct formula.
Check each atom or polyatomic ion to be sure the number is equal on both sides of the equation.
Make sure all the coefficients are in the lowest possible ratio.

8. Practice questions
We are going to start simple and work up to fairly complex.
Hydrochloric acid + Sodium hydroxide  Salt + Water
HCl(aq) + NaOH(aq)  NaCl(aq) H20(aq)
answer: This equation (reaction) is already balanced as a 1:1:1:1 ratio
Sulfuric acid + Potassium  Potassium sulphate + Hydrogen gas
H2SO4(aq) + K(s)  K2SO4(aq) + H2(g)
answer: 1. We treat the sulphate polyatomic ion as one unit.
2. Look at Hydrogen first. There are 2 on the reactant side and 2 on the product side so we are good for Hydrogen.
3. We have one polyatomic sulphate on the reactant side and one on the product side so we are good there too.
4. Now we come to Potassium. We have one on the reactant side but 2 on the product side. So we place a coefficient of 2 in front of the Potassium on the reactant side.
5. The balanced equation is: H2SO4(aq) + 2 K(s)  K2SO4(aq) + H2(g)
: : : 1

9. 3. Solid Aluminum is burned in the presence of Oxygen to form Aluminum Oxide. Al(s) + O2(g)  Al2O3(s) a. Note the odd number of Oxygen atoms in the product. b. Next balance the Aluminum by placing the coefficient of 2 in front of the Al(s) 2Al(s) + O2(g)  Al2O3(s) c. Multiply the Al2O3 by 2 to give an even number of Oxygen atoms on the product side. 2Al(s) + O2(g)  2Al2O3(s) d. Now we can add the coefficient of 3 in front of the O2(g) on the reactant side so we have the same number of Oxygen atoms on the reactant side and the product side. e. Rebalance the Al(s) on the reactant side to 4 to give us the final balanced equation of: 4Al(s) + 3O2(g)  2Al2O3(s) 4 : 3 : 2

10. Some more practice questions for your enjoyment
Balance each equation.
FeCl3 + NaOH  Fe(OH)3 + NaCl
CS2 + Cl2  CCl4 + S2Cl2
KI + Pb(NO3)2  PbI2 + KNO3
C2H2 + O2  CO2 + H2)
CO + Fe2O3  Fe + CO2

11. Reaction Assignment How do you write a skeleton equation? (2)
Describe the steps in writing a balanced equation. (5)
Balance the following: (8)
SO2(g) + O2(g)  SO3(g)
Fe2O3(s) + H2(g)  Fe(s) + H2O(l)
P(s) + O2(g)  P4O10(s)
Al(s) + N2(g)  AlN(s)